Ferro silicon is a kind of ferro alloys which is integrated of silicon and iron. The ratio of the two chemical substances are combined different, with the proportion of silicon ranging anywhere between 15% and 90% . The most widely used is 75% ferrosilicon. This alloy is also known by the name of ferro silicium. Its identification has known by CAS 8049-17-0 and its chemical formula is âFeSiâ.Because silicon and oxygen are easy to synthesize silicon dioxide, silicon iron is often used as deoxidizer when making steel, and because SiO2 produces a lot of heat, it also can improve the temperature of steel.